The present invention relates to a process and converter for the preparation of ammonia from an ammonia synthesis gas. In particular, the invention provides certain improvements in the ammonia synthesis process, wherein reacting synthesis gas is indirectly cooled with fresh synthesis gas in an ammonia converter provided with at least two catalyst beds of an ammonia synthesis catalyst.
By the inventive process, three separate streams of fresh ammonia synthesis gas are introduced into an ammonia converter provided with at least two catalyst beds connected in series,
a first scream of fresh synthesis gas is passed to a first gas mixing step;
a second stream of fresh syntheses gas is passed to a first indirect heat exchanging step with a first effluent stream from a first catalyst bed; and
a third stream of fresh synthesis gas is passed to a second heat exchanging step with a second effluent stream from the first catalyst bed;
the first and second effluent stream are provided by dividing partly converted synthesis gas stream from the first catalyst bed;
the first effluent stream is introduced into the first heat exchanging step and the second effluent steam into the second heat exchanging step and, thereby, the first and second stream of fresh synthesis gas are preheated by heat indirectly supplied from the first and second effluent stream;
the preheated first and second stream are passed to the mixing step, the preheated streams of fresh synthesis gas are mixed and combined with the first stream of fresh synthesis gas to obtain an ammonia process gasstream;
the process gas is introduced into the first catalyst bed and the partially converted synthesis gas stream is withdrawn;
the first and second effluent stream are combined to partly converted process gas; and
the partly converted process gas is introduced into at least the second catalyst bed to obtain a product stream being rich in ammonia.
In a sepcific embodiment of the invention the first bed is divided into a first part and second part with quenching of the effluent between the first and second part.
An ammonia converter for use in the above process comprises within a pressure shell at least a first and second catalyst bed being connected in series, a first and second heat exchanger arranged downstream the first catalyst bed, the first and second heat exchanger being connected in parallel and adapted to indirect heating of a first and second stream of fresh ammonia synthesis gas with a partly converted ammonia synthesis gas being withdrawn from the first catalyst bed and divided into a first substream and second substream and introduced into the first and second heat exchanger;
a first mixing device for mixing the first and second preheated stream of fresh ammonia gas with a third stream of fresh ammonia synthesis gas;
a second mixing device for mixing and combining the first and second substream of partly converted ammonia synthesis gas;
means for introducing the combined partly converted synthesis gas into at least the second catalyst bed.
The above ammonia synthesis process and ammonia converter according to a general object of the invention are, in particular, useful in the modernizing of existing ammonia processes and converters.
By the parallel connected heat exchanging steps and parallel heat exchangers, it will be possible to introduce fresh synthesis gas and to withdraw from the process product gas at a high temperature.
As a major advantage of the high outlet temperature obtained in the product gas of the invention, the product gas can be utilized as heating media in the preparation of valuable steam instead of boiler feed water as in the conventional ammonia synthesis process.